Gases and liquids are traditionally heated by shell and tube heat exchangers where a hot liquid or gas passing through the tubes provides the heat, which goes through the walls of the tubes, to heat the material passing through the heat exchanger on the exterior to the tubes. The shell contains the liquid or gas being heated and is usually cylindrical to provide a good pressure barrier. The pressure barrier at the ends of the cylinder is provided by a tube sheet into which the hollow tubes are swaged. However, many different designs are feasible. When the application is leak sensitive the exchanger is often provided with a double tube sheet with a gap between the tube sheets so that leaks can be prevented from going from the tube to the shell or vice versa and be observed so that repairs may be undertaken before a major leak occurs. As an alternative the heating fluid may be introduced in to the shell and the fluid to be heated may be passed through the interior of the tubes.
When greater temperatures are required than can be obtained from vapors, such as steam, or liquids used as thermal transfer fluids passing through the tubes, then electrical heaters are used in place of the tubes. However, electrical heaters present certain limitations compared to shell and tube heat exchangers. At least two basic designs are used: a furnace design where the fluid flows through tubes located inside an electrically heated furnace or a direct immersion design where the fluid flows over the heater rods which are directly inserted in a conduit of some kind.
One example of a furnace design is referred to as a radiant coil furnace (see Wellman design) in which a coiled pipe containing a gas is heated by electrical heater elements with the furnace walls containing the heat. The furnace usually has a lid or end plates through which the pipes protrude to make connection with the rest of the process. The pipes expand and move as they heat up. The furnace is not usually gas tight or pressure rated to allow for pipe movement and reduce cost.
A second example uses an immersion heater such as shown in U.S. Pat. No. 7,318,735 which is a flanged design in which multiple U-shaped heating elements are welded to a flange with wires connected to the electrical heaters extending out of the holes in the flange. The bundle of heater elements is placed inside an empty pipe and the liquid being heated enters and leaves from the side of the pipe.
Both types of design will release materials to atmosphere in the event of a leak in the tubes and will have to be shutdown for repairs. With corrosive materials the probability of the leak increases: many corrosive materials are also toxic thus providing a serious health hazard. Despite this leak potential, leak detection systems are not usually provided to warn the operator. Corrosion increases rapidly with temperature so any hot spots on the tube will corrode much faster. With the furnace design there is also some shadowing of parts of the tube so some parts are hotter than others. With the immersion design some areas may have poor flow and are thus unable to remove the heat and become hot spots. This is particularly the case with corrosive gases which are more difficult to heat.
It can be seen from FIG. 1 of U.S. Pat. No. 7,318,735 that the fluid comes in from the side and thus must turn to go down and out the exit. Such changes in direction create areas of low flow in the transition from cross flow to axial flow which can create hot spots. In the '735 patent there is no mechanism to aid in this transition. Also, it is a characteristic of electrical heaters that the heat emitted per unit length is constant; thus, if this heat is not removed evenly from the whole area of the heater, “hot spots” can develop. This is not the case for shell and tube heat exchangers as areas of low heat transfer simply do not transfer heat thus the hot spot problem is much less severe. Thus it is not possible to use standard shell and tube designs with electrical heat as the typical cross flow baffles cause hot spots. Also it can be seen that the failure of one heater tube or wire requires removal of the entire assembly to repair the failure. This adds to the cost of operation as is discussed in U.S. Pat. No. 7,318,735. However, the solution presented therein also has problems in that the unit must be shutdown and dismantled in order to weld on the header plate.
A further problem with corrosive materials is that they typically have an upper temperature which should not be exceeded. This then limits the flux which may be used at the hot end of the heater. However, since heaters typically have a single flux this can mean there is also a low flux at the cold end and thus the overall heater is much bigger. One solution to this is a variable flux rate where the flux is higher at the cold end than at the hot end, but such heaters are more expensive to make and are not readily available. A further disadvantage is the absence of methods to measure the heater temperature and thus be aware if a heater is overheating. It is possible to put separate thermowells through the header plate but this requires more room and additional penetrations of the plate and each thermowell only measures the point on the heater that it contacts.